Hydraulic circuit for actively controlled automotive suspension system

ABSTRACT

An actively controlled suspension system includes a hydraulic circuit connecting a pressure source unit to a pressure control valve unit which controls working fluid pressure in a working chamber of a hydraulic cylinder serving as principle component of a suspension system for supporting a vehicle body on a suspension member. The hydraulic circuit includes a supply line for supplying a pressurized fluid to the pressure control valve and a drain line for draining the pressurized fluid from the pressure control valve. A one-way check valve is disposed between the pressure source and the pressure control valve for presenting surge flow of the working fluid from the pressure control valve unit to the pressure source. An operational one-way check valve is disposed in the drain line for preventing surge flow from the pressure source to the pressure control valve unit. The operational one-way check valve is connected to the supply line downstream of the one-way check valve as a pilot pressure so as to permit fluid flow from the pressure control valve unit to the pressure source unit when the working fluid pressure in the drain line is higher than the pilot pressure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an actively controlled suspension system for an automotive vehicle, which controls suspension characteristics depending upon vehicle driving condition in order to suppress attitude change of a vehicular body. More specifically, the invention relates to a hydraulic circuit construction associated with an actively controlled suspension system which can prevent the suspension system from causing abrupt or sudden change of vehicular attitude upon ON-set of pressure source unit from inactive state.

2. Description of the Background Art

U.S. Pat. No. 4,702,490, issued on Oct. 27, 1987 which has been assigned to the common owner to the present invention, discloses one of typical construction of an actively controlled suspension system, in which a hydraulic cylinder defining a working chamber is disposed between a vehicular body and a suspension member rotatably supporting a vehicular wheel. The working chamber of the hydraulic cylinder is communicated with a hydraulic circuit including a pressurized working fluid source. A pressure control valve, such as an proportioning valve assembly, is disposed in the hydraulic circuit, which is connected to an electric or electronic control circuit to be controlled the valve position. The pressure control valve is controlled the valve position by a suspension control signal produced in the control circuit for adjusting pressure in the working chamber and whereby controlling suspension characteristics.

On the other hand, European Pat. Nos. 0 283 004, 0 285 153 and 0 284 053 discloses technologies for controlling the suspension systems constructed as set forth above, depending upon the vehicle driving condition for suppressing rolling and/or pitching of the vehicular body.

In one of the typical construction of the hydraulic circuit includes a pressure source unit which comprises a fluid pump drivingly associated with an automotive internal combustion engine so as to be driven by the engine output torque. This means that, when the engine is held inactive, the fluid pump cannot be driven for generating fluid pressure in the working fluid. Therefore, in response to turn OFF the engine, the hydraulic circuit becomes inactive to drop pressure to zero. This may cause variation of the fluid pressure in the working chamber of the hydraulic cylinder to cause lowering of the vehicular height. This is totally acceptable for degradation of the riding comfort and also for possibility of damaging of the vehicle body by causing contact with a step on the road.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide an improved hydraulic circuit which can solve the aforementioned and other drawback in the prior proposed actively controlled suspension system.

Another object of the invention to provide an actively controlled suspension system with a hydraulic circuit which has a capability of holding line pressure at a predetermined level for preventing the suspension system from causing substantial change of vehicular attitude depending on active state and inactive state of a pressure source unit.

In order to accomplish aforementioned and other objects, an actively controlled suspension system, according to the present invention, includes a hydraulic circuit connecting a pressure source unit to a pressure control valve unit which controls working fluid pressure in a working chamber of a hydraulic cylinder serving as principle component of a suspension system for supporting a vehicle body on a suspension member. The hydraulic circuit includes a supply line for supplying a pressurized fluid to the pressure control valve and a drain line for draining the pressurized fluid from the pressure control valve. A one-way check valve is disposed between the pressure source and the pressure control valve for presenting surge flow of the working fluid from the pressure control valve unit to the pressure source. An operational one-way check valve is disposed in the drain line for preventing surge flow from the pressure source to the pressure control valve unit. The operational one-way check valve is connected to the supply line downstream of the one-way check valve as a pilot pressure so as to permit fluid flow from the pressure control valve unit to the pressure source unit when the working fluid pressure in the drain line is higher than the pilot pressure.

According to one aspect of the invention, a hydraulic circuit for an actively controlled suspension system which includes a pressure control valve unit connected to a working chamber in a hydraulic cylinder disposed between a vehicle body and a suspension member rotatable supporting a road wheel, the pressure control valve having an inlet port, a pressure control port and a drain port and connected to the working chamber via the pressure control port for adjusting fluid pressure in the working chamber in order to control suspension characteristics according to the vehicle driving condition, the hydraulic circuit being associated with the pressure control valve unit for supplying a pressurized fluid, which hydraulic circuit comprises:

a fluid pressure source unit for generating a pressurized fluid and including a high pressure section for supplying the pressurized fluid and a low pressure section for storing the working fluid;

a supply line connecting the high pressure section of the fluid pressure source unit to the inlet port of the pressure control valve unit for supplying the pressurized fluid;

a drain line connecting the drain port of the pressure control valve unit to the low pressure section of the fluid pressure source unit;

a valve means disposed within the drain line and variable of operational mode depending upon a line pressure in the supply line, the valve means operating in a first mode to perform pressure relief operation for draining a fluid pressure in the drain line higher than a predetermined pressure while the fluid pressure in the drain line upstream thereof is higher than the line pressure, and operating in a second mode to perform checking operation for switching valve position between open position and closed position when the line pressure in the supply line varies across the predetermined pressure,

Preferably, the pressure control valve unit may be adapted to control fluid pressure in the working chamber in a range between a predetermined maximum pressure and a predetermined minimum pressure across a predetermined neutral pressure, at which the relative distance between the vehicle body and the suspension member is maintained at a predetermined standard distance at a standard load condition, and the valve means may be set the predetermined pressure at a value corresponding to the neutral pressure of the pressure control valve unit. The valve means may comprise a valve means having a first and second components which are operative independent of each other in the first mode and cooperative with each other in the second mode. On the other hand, the valve means may comprise a valve housing defining a valve bore in which the first and second components are thrustingly disposed, the first and second components being cooperative with the inner periphery of the valve housing to define a first chamber communicated with the supply line via a pilot line, a second chamber disposed between the drain port of the pressure control valve unit and the low pressure section of the pressure source unit, the second chamber being defined between the first and second components and the first chamber is defined at a side of the first component remote from the second chamber so that the operational mode of the valve means is switched between the first mode and the second mode depending upon the force balance of the fluid force exerted on both sides of the first component.

In the preferred construction, the second component is associated with a initial force setting means acting on the side remote from the first chamber, the initial force setting means exerting the predetermined pressure to the second component. The initial force setting means comprises a mechanical spring.

According to another aspect of the invention, in a hydraulic circuit for an actively controlled suspension system which includes a pressure control valve unit connected to a working chamber in a hydraulic cylinder disposed between a vehicle body and a suspension member rotatable supporting a road wheel, the pressure control valve having an inlet port, a pressure control port and a drain port and connected to the working chamber via the pressure control port for adjusting fluid pressure in the working chamber in order to control suspension characteristics according to the vehicle driving condition, the hydraulic circuit being associated with the pressure control valve unit for supplying a pressurized fluid, and the hydraulic circuit including a fluid pressure source unit for generating a pressurized fluid and including a high pressure section for supplying the pressurized fluid and a low pressure section for storing the working fluid, a supply line connecting the high pressure section of the fluid pressure source unit to the inlet port of the pressure control valve unit for supplying the pressurized fluid, a drain line connecting the drain port of the pressure control valve unit to the low pressure section of the fluid pressure source unit, a valve means, disposed within the drain line, comprises:

first means, associated with the valve means, for varying operational mode of the valve means depending upon a line pressure in the supply line, the first means operating the valve means in a first mode while the fluid pressure in the drain line upstream of the valve means is higher than the line pressure, and operating in a second mode while the pressure in the drain line upstream of the valve means is lower than or equal to the pressure in the supply line;

a second means, active during the first mode of the valve means for performing pressure relief operation for draining a fluid pressure in the drain line higher than a predetermined pressure; and

a third means, active during the second mode of the valve means, for performing fluid flow checking operation for switching valve position between open position and closed position.

The pressure control valve unit may be adapted to control fluid pressure in the working chamber in a range between a predetermined maximum pressure and a predetermined minimum pressure across a predetermined neutral pressure, at which the relative distance between the vehicle body and the suspension member is maintained at a predetermined standard distance at a standard load condition, and the valve means is set the predetermined pressure at a value corresponding to the neutral pressure of the pressure control valve unit. The first means comprises a valve member having a first and second components which are operative independent of each other in the first mode and cooperative with each other in the second mode. The second means may be active on the first component for shifting the latter depending upon the fluid pressure in the drain line for relieving fluid pressure in excess of the predetermined pressure. In the preferred construction, the third means is active on the first and second components in a condition cooperating to each other, for establishing fluid communication between the drain port of the pressure control valve unit and the low pressure section of the pressure source unit when the pressure in the supply line is higher than the predetermined pressure and blocking fluid communication between the drain port of the pressure control valve unit and the low pressure section of the pressure source unit when the pressure in the supply line is lower than or equal to the predetermined pressure.

According to a further aspect of the invention, an actively controlled suspension system comprises:

a hydraulic cylinder disposed between a vehicle body and a suspension member rotatable supporting a road wheel and defining a working chamber filled with a hydraulic working fluid;

a hydraulic circuit includes a pressure control valve unit connected to the working chamber, the pressure control valve having an inlet port, a pressure control port and a drain port and connected to the working chamber via the pressure control port for adjusting fluid pressure in the working chamber in order to control suspension characteristics according to the vehicle driving condition;

a fluid pressure source unit for generating a pressurized fluid and including a high pressure section for supplying the pressurized fluid and a low pressure section for storing the working fluid;

a supply line connecting the high pressure section of the fluid pressure source unit to the inlet port of the pressure control valve unit for supplying the pressurized fluid;

a drain line connecting the drain port of the pressure control valve unit to the low pressure section of the fluid pressure source unit;

a valve means disposed within the drain line and variable of operational mode depending upon a line pressure in the supply line, the valve means operating in a first mode to perform pressure relief operation for draining a fluid pressure in the drain line higher than a predetermined pressure while the fluid pressure in the drain line upstream thereof is higher than the line pressure, and operating in a second mode to perform checking operation for switching valve position between open position and closed position when the line pressure in the supply line varies across the predetermined pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood from the detailed description given herebelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken for limiting the invention to the specific embodiments, but are for explanation and understanding only.

In the drawings:

FIG. 1 is a diagrammatical illustration of the overall construction of the preferred embodiment of an actively controlled suspension system, according to the present invention, in which the preferred embodiment of a proportioning valve assembly is employed as a pressure control valve;

FIG. 2 is a sectional view of the preferred embodiment of the pressure control valve according to the present invention;

FIG. 3 is a circuit diagram of one example of hydraulic circuit which is applicable for the actively controlled suspension system according to the present invention;

FIG. 4 is a chart showing relationship between an electric current value of a control signal to be supplied for an actuator of the pressure control valve and a working fluid pressure supplied to a working chamber of a hydraulic cylinder; and

FIG. 5 is a sectional view of an operational one-way check valve employed in the preferred embodiment of the hydraulic circuit of the actively controlled suspension system of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, particularly to FIG. 1, the preferred embodiment of an actively controlled suspension system, according to the present invention, is designed to generally perform suspension control for regulating vehicular height level and vehicular attitude by suppressing relative displacement between a vehicular body 10 and suspension members 24FL, 24FR, 24RL and 24RR provided in front-left, front-right, rear-left and rear-right suspension mechanism 14FL, 14FR, 14RL and 14RR and rotatably supporting front-left, front-right. rear-left and rear-right wheels 11FL, 11FR, 11RL and 11RR. The suspension member will be hereafter represented by the reference numeral "24" as generally referred to. Similarly, the suspension mechanism as generally referred to will be hereafter represented by the reference numeral "14" Respective front-left, front-right, rear-left and rear-right suspension mechanisms 14FL, 14FR, 14RL and 14RR have hydraulic cylinders 26FL, 26FR, 26RL and 26RR which will be represented by the reference numeral "26" as generally referred to.

Each of the hydraulic cylinder 26 is disposed between the vehicular body 10 and the suspension member 24 to produce a damping force for suppressing relative displacement between the vehicular body and the suspension member. The hydraulic cylinder 26 generally comprises an essentially enclosed cylindrical cylinder body 26a defining therein an enclosed chamber. A thrusting piston 26c is thrustingly and slidably disposed within the enclosed chamber of the hydraulic cylinder 26 for defining in the latter a working chamber 26d and a reference pressure chamber 26e. The working chamber 26d may be communicated with the reference pressure chamber 26e via an orifice formed through the piston for fluid communication therebetween in an substantially restricted amount. The piston 26c is connected to the associated one of suspension member 24 via a piston rod 26b. A suspension coil spring 36 employed in the shown type of the suspension system is not required a resilient force in a magnitude required in the ordinary suspension system and only required the resilient force necessary for maintaining the vehicular body about the suspension member.

The working chamber 26d of the hydraulic cylinder 26 is connected one of pressure control valves 28FL, 28FR, 28RL and 28RR via a pressure control line 38. The pressure control valve 28FL, 28FR, 28RL and 28RR will be hereafter represented by the reference numeral "28" as generally referred to. The pressure control valve 28 is, in turn, connected to a pressure source unit 16 via a supply line 35 and a drain line 37. A branch circuit is provided for connecting the pressure control line 38 to a pressure accumulator 34 via a flow restricting means, such as an orifice 32. Another pressure accumulator 18 is provided in the supply line 35 for accumulating the excessive pressure generated in the pressure source unit 16.

The pressure control valves 28 comprise, though it is not clearly shown in FIG. 1, electrically or electromagnetically operable actuators (reference is made to FIG. 2), such as a proportioning solenoids. The actuators are connected to a microprocessor based control unit 22. The control unit 22 is connected a plurality of vehicular height sensors 21 which are disposed in respectively associated suspension mechanism and designed for monitoring relative position of the vehicular body 10 and the relevant suspension member 24 to produce a vehicular height level indicative signals h₁, h₂, h₃ and h₄. The control unit 22 is also connected to a lateral acceleration sensor 23, a longitudinal acceleration sensor 25 and so forth to receive the vehicle driving condition indicative parameters. Based on these control parameters, including the height level indicative signals, a lateral acceleration indicative signal G_(y) generated by the lateral acceleration sensor, a longitudinal acceleration indicative signal G_(x) generated by the longitudinal acceleration sensor, and so forth, the control unit performs anti-rolling, anti-pitching and bouncing suppressive suspension controls.

While the specific sensors, such as the vehicle height sensors which comprise strike sensor, the lateral acceleration sensor 23 and the longitudinal acceleration sensor 25, it is possible to replace or add any other sensors which monitors vehicle driving parameter associated with the suspension control. For instance, the stroke sensors employed in the shown embodiment can be replaced with one or more vertical acceleration sensors. Similarly, the lateral acceleration sensor may be replaced with a steering angle sensor for monitoring steering behaviour for assuming lateral force to be exerted on the vehicular body. In the later case, the parameter indicative of the steering angular displacement may be used in combination of a vehicular speed data since vehicular speed may influence for rolling magnitude of the vehicle during steering operation.

As shown in FIG. 2 in detail, the pressure control valve 28 comprises a proportioning valve assembly and is designed to be controlled by an electric current as a control signal supplied from the control unit 22 for varying valve position according to variation of the current value of the control signal. Generally, the pressure control valve 28 controls magnitude of introduction and draining of pressurized fluid into and from the working chamber 26d for adjusting the pressure in the working chamber. As will be appreciated, since the adjusted fluid pressure in the working fluid determines damping force to be created in response to relative displacement between the vehicle body 10 and the suspension member 24. Mode of the suspension mechanism is varied according to variation of the fluid pressure in the working chamber between a predetermined hardest mode to most soft mode.

In the construction of the pressure control valve shown in FIG. 2, the pressure control valve 28 includes a valve housing 42. The valve housing 42 defines a valve bore 44 which is separated in to a valve chamber 42L and a control chamber 42U by means of a partitioning wall 46. The partitioning wall 46 is formed with a communication path opening 46A for communication between the control chamber 42U and the valve chamber 42L. As seen from FIG. 2, the control chamber 42U and the valve chamber 42L are arranged in alignment to each other across the communication path opening 46A. In parallel to a section of the partitioning wall 46 extending perpendicular to the axis of the valve chamber 42L and the control chamber 42U, a fixed orifice defining partitioning member 50 is provided. The partitioning member 50 defines a throttling orifice which is oriented substantially in alignment with the communication path opening 46A. The partitioning wall 46 and the partitioning member 50 are cooperative for defining a pilot chamber PR therebetween.

A valve spool 52 is thrustingly and slidingly disposed within the valve chamber 42L. The valve spool 52 defines an upper feedback chamber FU between the tip end thereof and the partitioning member 50. The valve spool 52 also defines a lower feedback chamber FL between the other tip end thereof and the bottom of the valve chamber 42L. Offset springs 60U and 60L are disposed within the upper and lower feedback chambers FU and FL, which offset springs exerts spring force to the valve spool 52 for resiliently restricting movement of the latter. Resilient force of the offset springs 60U and 60L are so set as to balance to place the valve spool 52 at a neutral position, when the fluid pressure in the upper and flow feedback chambers FU and FL balances to each other. The valve chamber 42L is communicated with a supply line 35 via an supply port 54s, a drain line 37 via a drain port 54r and a pressure control line 38 via a control port 54c, which supply port, drain port and control port are defined in the valve housing 42. The valve spool 52 at the aforementioned neutral position, blocks fluid communication of the pressure control chamber PC with any of the supply port 54s and the drain port 54r. As a result, as long as the valve spool 52 is maintained at the neutral position, overall fluid force in the hydraulic circuit downstream of the pressure control valve, which circuit includes the working chamber 26d of the hydraulic cylinder 26 is held constant.

The valve spool 52 is formed with lands 52a and 52b connected to each other via smaller diameter bar-like section 52e. The land 52a is oriented adjacent the lower feedback chamber FL so as to subject the tip end to the fluid pressure in the lower feedback chamber. Similarly, the land 52b is oriented adjacent the upper feedback chamber FU so as to subject the tip end to the fluid pressure in the upper feedback chamber. The bar-like section 52e between the lands 52a and 52b is cooperative with the peripheral wall of the valve chamber 42L in order to define therebetween a pressure control chamber PC. A fluid flow path 52d is formed through the valve spool 52. The fluid flow path 52d has one end communicated with the pressure control chamber PC and the other end communicated with the lower feedback chamber FL. A fixed flow restricting orifice 52f is formed in the fluid flow path 52d for restricting fluid flow therethrough.

A poppet valve member 48 is disposed within the control chamber 42U for thrusting movement therein. The poppet valve member 48 has a valve head 48a of an essentially conical configuration. The valve head 48a opposes to the communication path opening 46A of the partitioning wall 46. The poppet valve member 48 is operably associated with a proportioning solenoid assembly 29 as the actuator. The proportioning solenoid assembly 29 comprises a housing 62 rigidly secured on the valve housing 42 and defining an internal space to receive therein a plunger 66. The plunger 66 has a plunger rod 66A. The tip end of the plunger rod 66A is kept in contact with the tip end of the poppet valve member 48 remote from the valve head 48a. Therefore, the poppet valve member 48 is axially driven by means of the plunger 66 to control the path area in the communication path opening 46A according to the position of the tip end of the plunger rod 66A. Adjusting of the path area in the communication path opening 46A results in variation of fluid pressure to be introduced into the pilot chamber PR.

In order to control the position of the plunger 66 with the plunger rod 66A, a proportioning solenoid coil 68 is housed within the housing 62 and surrounds the plunger 66. The interior space of the housing 62 is connected to the control chamber 42U for fluid communication therebetween. The plunger 66 is formed with a fluid path 66B for fluid communication between upper and lower sections of the interior space. Therefore, the fluid pressure in the upper and lower sections of the interior space of the housing 62 is held equal to the pressure in the control chamber 42U. This cancels fluid pressure to be exerted on the poppet valve and the plunger so that the position of the tip end of the plunger rod 66A can be determined solely depending upon the magnitude of energization of the proportioning solenoid coil 68.

As seen from FIG. 2, the poppet valve member 48 has a cylindrical larger diameter section 48b for separating the control chamber 42U into upper section and lower section 42Uu and 42Ul. The upper and lower sections 42Uu and 42Ul are communicated with the drain port 54r via a pilot return path PT. A multi-stage orifice Pr is provided in the pilot return path PT for restricting fluid flow therethrough. The multi-stage orifice Pr comprises a plurality of strips formed with through openings and is so designed that one of the orifice oriented at most upstream side is mainly effective for restricting fluid flow when fluid flowing therethrough is steady flow and that all of the orifices of respective strips are equally effective for restricting fluid flow when fluid flow therethrough is disturbed and not steady. Therefore, as will be appreciated herefrom, the multi-stage orifice Pr employed in the shown embodiment serves to provide greater fluid flow restriction against non-steady or disturbed fluid flow than that for the steady flow. As seen from FIG. 2, the multi-stage orifice Pr is provided upstream of the upper and lower sections 42Uu and 42Ul. On the other hand, a fixed throttling orifice Pd is provided at an orientation downstream of the lower section 42Ul and upstream of the upper section 42Uu. Similarly, the pilot chamber PR is communicated with the supply port 54s via a pilot path PP. A multi-stage orifice Qp which has similar construction and flow restricting function to that of the multi-stage orifice Pr is provided in the pilot path PP.

A fixed throttle orifice Pro is also provided in the drain port 54r for restricting fluid flow therethrough. The diameter of the fluid path at the orifice Pro is so selected as to create great flow restriction against pulsatile fluid flow cyclically varying the fluid pressure at a frequency approximately 1 Hz.

As can be seen from FIG. 2, the pressure control valve 28 is so arranged as to direct the axis of the valve bore 44 parallel to the longitudinal axis of the vehicle body. The longitudinal acceleration to be exerted on the vehicular body is much smaller than the lateral acceleration and vertical acceleration exerted on the vehicle body. Therefore, by arranging the pressure control valve 28 so that the poppet valve 48 and the valve spool 52 thrustingly move in longitudinal direction, influence of the externally applied acceleration can be minimized.

FIG. 3 shows detailed circuit construction of one example of hydraulic circuit which is applicable for the shown embodiment of the actively controlled suspension system, according to the present invention. The pressure source unit 16 includes the pressure unit 16b which comprises a fluid pump, and is connected to a fluid reservoir 16a via a suction pipe 201 The fluid pump 16b is associated with an automotive engine 200 so as to be driven by the output torque of the latter output from an engine output shaft 200a. The outlet of the pressure unit 16b, through which the pressurized working fluid is discharged, is connected to the supply port 54s of the pressure control valve 28 via the supply line 35. An one-way check valve 220, a pressure accumulator 222 for absorbing pulsatile, a filter 224 are disposed in a portion 35a of the supply line 35. A by-pass passage 226 with an one-way check valve 228 is provided for by-passing the filter 224.

A pressure accumulators 27 are also connected to the supply line 35 to receive therefrom the pressurized fluid for accumulating the pressure. An one-way check valve 204 is disposed in the supply line 35 at the position upstream of the junction between the pressure accumulators 27 and the supply line 35.

A pressure relief line 205 is also connected to the supply line 35 at the position intermediate between the filter 224 and the one-way check valve 204, at one end. The other end of the pressure relief line 205 is connected to the drain line 37. A pressure relief valve 206 is disposed in the pressure relief line 205. The pressure relief valve 206 is responsive to the fluid pressure in the supply line 35 higher than a given value to drain port of the working fluid to the drain line for maintaining the pressure in the supply line 35 below the given pressure value.

On the other hand, an operational one-way check valve 300 is disposed in the drain line 37. The operational one-way check valve 300 is also connected to the supply line 35 at downstream of the one-way check valve 204 to receive therefrom the pressure in the supply line as a pilot pressure, via a pilot line 208. The operational one-way check valve 300 is designed to be maintained at open position as long as pilot pressure introduced from the supply line 35 at the orientation downstream of the one-way check valve 204 is held higher than a predetermined pressure. At the open position, the operational one-way check valve maintains fluid communication between the inlet side and outlet side thereof so that the working fluid in the drain line 37 may flow therethrough to the reservoir tank 16a. On the other hand, the operational one-way check valve 300 is responsive to the working fluid pressure in the supply line downstream of the one-way check valve 204 serving as the pilot pressure dropping below the predetermined pressure level to be switched into shut-off position. At the shut-off position, the operational one-way check valve 300 blocks fluid communication between the drain port 54r of the pressure control valve 28 and the reservoir tank 16a. In the shown embodiment, the predetermined pressure is set at a pressure corresponding to the neutral pressure of the pressure control valve unit 28.

An oil cooler 211 is disposed in the drain line 37 for cooling the working fluid returning to the reservoir tank 16a.

FIG. 5 shows the detailed construction of the preferred embodiment of the operational one-way check valve 300 to be employed in the preferred embodiment of the actively controlled suspension system according to the present invention. As shown in FIG. 5, the operational one-way check valve 300 comprises a valve housing 302 formed with an inlet port 304, an outlet port 306 and a pilot port 308. The valve housing 302 defines a valve bore 310. The valve bore 310 comprises a larger diameter section 312, in which a poppet valve 314 is thrustingly disposed, and a smaller diameter section 316, in which a valve spool 318 is disposed. The pilot port 308 is communicated with the supply line 35 at the section 35a disposed between the one-way check valve 204 and the pressure control valve unit 28FL 28FR, 28RL and 28RR, via the pilot line 300a. The pilot port 308 is, on the other hand, communicated with the smaller diameter section 316 to supply the line pressure of the supply line 35 at the orientation downstream of the one-way check valve 204 as the pilot pressure Pp. On the other hand, the inlet port 304 is communicated with the drain port 54r of the pressure control valve unit 28 via a section 37b of the drain line 37. The inlet port 304 communicates with the smaller diameter section 316 via an annular groove 324 formed on the inner periphery of the valve housing 302. The outlet port 306 is communicated with the fluid reservoir 16a via a section 37a of the drain line 37 and, in turn, communicated with the larger diameter section 312 via an annular groove 326 formed on the inner periphery of the valve housing 302. As seen from FIG. 5, the annular grooves 324 and 326 are oriented in side-by-side relationship with leaving a radially and inwardly projecting land 328. The land 328 has a shoulder 330.

The valve spool 318 and the poppet valve 314 are cooperated with each other to define therebetween a control chamber 334 which communicates with the inlet port 304 and the outlet port 306. On the other hand, the valve spool 318 also defines a pilot chamber 336 at a side remote from the control chamber 334. The poppet valve 314 defines a pressure setting chamber 338 at a side remote from the control chamber 334. The pressure setting chamber 338 is communicated with the outlet port 306 via a path 340. A set spring 342 is disposed within the pressure setting chamber 338 for normally exerting a spring force to the poppet valve 314. In the preferred embodiment, the set spring 342 is provided a set force which corresponds the neutral pressure P_(N) of the pressure control valve unit 28.

The valve spool 318 has a valve body 320 and a valve stem 322 projecting from the valve body toward the poppet valve 314. The tip end of the valve stem 322 contacts with the mating surface of the poppet valve 314. The poppet valve 314 has an annular shoulder 332 mating with the shoulder of the land 330.

With the construction set forth above, the operational one-way check valve 300 operates as both of the pressure relief valve for relieving the excessive pressure in the drain line and one-way check valve. The relief pressure of the poppet valve 314 can be illustrated by the following balancing equation:

F₀ =P_(p0) ×A

where F₀ is the set pressure of the set spring 342;

A is an effective area of the spool; and

P_(p0) is a relief pressure.

Here, assuming that the pressure pi at the inlet port 304 is greater than or equal to the pilot pressure Pp at the pilot chamber 336, the valve spool 318 is shifted away from the poppet valve 314 so that the pilot pressure Pp in the pilot chamber 336 is not active on the valve position of the poppet valve. In such case, the poppet valve 314 operates purely as the pressure relief valve for relieving excessive pressure. At this time, the force balance as illustrated by:

    Pi×A=P.sub.p0 =A

can be established/ Therefore, as long as the fluid pressure at the inlet port 304 is higher than the relief pressure P_(p0), the shoulder 332 of the poppet valve 314 is held away from the shoulder 330 of the land 328 so as to permit fluid flow through the outlet port 306 and the section 37a of the drain line 37 to the fluid reservoir 16a. On the other hand, when the pressure at the inlet port 304 is lower than or equal to the relief pressure P_(p0), then, the spring force of the set spring 342 overcomes the fluid pressure to establish contact between the mating shoulders 332 and 330 to block fluid communication between the control chamber 334 and the outlet port 306.

On the other hand, when the pressure Pi at the inlet port 304 is lower than the pilot pressure P_(p) in the pilot chamber 336, the valve spool 318 is shifted toward the poppet valve 314 to contact with the latter at the tip end of the valve stem 334. At this time, the force to depress the valve stem 334 onto the poppet valve 314 can be illustrated by (P_(p) -Pi)×A. At this time, the pressure Pi introduced into the control chamber 334 via the inlet port 304 is canceled as an internal pressure. Therefore, the pressure balance at the poppet valve 314 can be illustrated by:

    F.sub.0 +k.sub.x =P.sub.p ×A

where k is a spring coefficient of the set spring 342; and

x is a stroke of the poppet valve 314.

From the balancing equations give hereabove, the operational check valve 300 becomes open when the pilot pressure P_(p) is higher than the relief pressure P_(p0) and is held at shut-off position while the pilot pressure is held lower than the relief pressure.

In the hydraulic circuit set forth above, the fluid is pump 16 is driven by the engine 200 to discharge pressurized working fluid while the engine is running. The pressurized fluid discharged from the outlet of the fluid pump 16 is fed to the pressure control valve 28 via the supply line 35 including the pressure regulating orifice 202 and the one-way check valve 204. When the pressure control valve 28 is shifted to establish fluid communication between the supply port 54s and the pressure control port 54c from the valve position shown in FIG. 2, the pressurized working fluid passes the pressure control valve 28 and introduced into the working chamber 26d of the hydraulic cylinder 26. On the other hand, when the pressure control valve 28 is shifted to block fluid communication between the supply port 54s and the pressure control chamber PC, the fluid pressure in the supply line 35 increases. When the line pressure in the supply line 35 becomes higher than or equal to the set pressure of the pressure relief valve 206 in the pressure relief line 205, the excessive pressure is fed to the drain line 37 via the pressure relief valve 206 and thus returned to the reservoir tank 16a.

The fluid pressure in the supply line 35 is also fed to the operational one-way check valve 300 via a pilot line 208. As set forth, the operational one-way check valve 300 is placed at open position as long as the pilot pressure introduced through the pilot line 300a is held higher than or equal to the set pressure thereof. Therefore, fluid communication between the pressure control valve 28 and the reservoir tank 16a is maintained. At this position, the working fluid is thus returned to the reservoir tank 16a via the drain line 37 via the operational one-way check valve 300 and the oil cooler 211.

The operational one-way check valve 300, even at the open position, serves as a resistance to the fluid flow. Therefore, the fluid pressure in the drain line 37 upstream of the operational one-way check valve 300 becomes higher, i.e. higher than the offset pressure P₀. Then, the pressure relief valve 209 becomes active to open for allowing the excessive pressure of the working fluid to flow through the by-pass line 210.

When the engine stops, the pressure unit 16 ceases operation. By stopping the pressure unit 16, the working fluid pressure in the supply line 35 drops. According to drop of the pressure in the supply line 35, the pilot pressure to be exerted to the operational one-way check valve 300 via the pilot line 300a drops. When the pressure in the pilot line 300a drops below or equal to the set pressure, the operational one-way check valve 300 is switched into operational one-way check position to block fluid communication therethrough. As a results, the fluid pressure in the drain line 37 upstream of the operational one-way check valve 300 becomes equal to the pressure in the working chamber 26d. Therefore, even when the working fluid leaks through a gap between the spool valve 52 and the inner periphery of the valve bore, it is not affect the fluid pressure in the working chamber.

FIG. 4 shows variation of the working fluid pressure in the working chamber 26d of the hydraulic cylinder 26 according to variation of the current value of the control signal applied to the actuator 29 of the pressure control valve 28. As seen from FIG. 4, the hydraulic pressure in the working chamber 26d varies between a maximum pressure P_(max) which is saturation pressure of the pressure source unit 16 and a minimum pressure P_(min) which is set at a magnitude in view of a noise component to be contained in the control signal. As seen from FIG. 4, the maximum pressure P_(max) corresponds to the maximum current value I_(max) of the control signal and the minimum pressure P_(min) corresponds to the minimum current value I_(min) of the control signal. Furthermore, the hydraulic pressure level as labeled P_(N) represents neutral pressure at the neutral current I_(N). As seen, the neutral current I_(N) is set at an intermediate value between the maximum and minimum current values I_(max) and I_(min).

Operation of the aforementioned pressure control valve 28 in terms of control of suspension characteristics and absorption of road shock will be discussed herebelow.

In general, the pressurized working fluid source unit 16 supplies the predetermined line pressure. For example, the line pressure in the supply line 35 may be set at a pressure of 80 kgf/cm².

When the vehicle steadily travels on a smooth straight road, the current value of the control signal to be applied to the actuator 29 of the pressure control valve 28 is maintained at the neutral value I_(N). As long as the neutral value I_(N) of the control signal is applied to the actuator 29, the proportioning solenoid coil 68 is energized at a magnitude corresponding the neutral value I_(N) of the control signal to place the poppet valve 48 at the corresponding position. At this position, the flow resistance at the communication path opening 46A, path area of which is restricted by the valve head 48a of the poppet valve 48 becomes the neutral value. At this position of the poppet valve 48, the pilot pressure P_(p) within the pilot chamber PR is maintained at the neutral pressure P_(N). At this condition, if the fluid pressures is the control pressure Pc in the pressure control port 54c is held equal to the fluid pressure in the working chamber 26d of the hydraulic cylinder 26, the fluid pressure in the upper and lower feedback chambers FU and FL are held in balance to each other. The valve spool 52 is maintained at the neutral position to shut fluid communication between the supply port 54s, the drain port 54r and the pressure control port 54c. Therefore, the control pressure Pc is maintained at the neutral pressure P_(N).

At this condition, when relatively high frequency and small magnitude road shock input through the vehicular wheel, is absorbed by fluid communication between the working chamber 26d and the pressure accumulator 34 via the orifice 32. The flow restriction in the orifice 32 serves to absorb the bounding and rebounding energy. Therefore, high frequency and small magnitude road shock can be effectively absorbed so as not to be transmitted to the vehicle body.

When the piston 26c strokes in rebounding direction compressing the working chamber 26d, the fluid pressure in the working chamber increases to increase the control pressure Pc in the pressure control port 54c. Therefore, the control pressure Pc becomes higher than the pilot pressure Pp in the pilot chamber PR. This results in increasing of the fluid pressure in the lower feedback chamber FL at a magnitude higher than that in the upper feedback chamber FU. This causes upward movement of the valve spool 52 to establish fluid communication between the drain port 54r and the pressure control port 54c. Therefore, the pressure in the pressure control port 54c is drained through the drain line 37. This causes pressure drop at the pressure control port 54c so that the control pressure Pc becomes lower than the pilot pressure pp in the pilot chamber PR. Then, the fluid pressure in the upper feedback chamber FU becomes higher than that in the lower feedback chamber FL. Therefore, the valve spool 52 is shifted downwardly to establish fluid communication between the supply port 54s and the pressure control port 54c. The pressurized working fluid in the supply line 35 is thus supplied to the working chamber 26d via the pressure control port 54c to increase the fluid pressure. By repeating the foregoing cycles, pressure balance is established between the pressure control port 54c and the pilot chamber PR. Therefore, the control pressure pc as well as the fluid pressure in the working chamber 26d are adjusted to the pilot pressure.

During the pressure adjusting operation set forth above, the fixed throttling orifice Pro serves for restricting fluid flow from the pressure control port 54c to the drain line 37. This flow restriction at the orifice Pro serves as resistance against the rebounding stroke of the piston 26c to damp or absorb energy causing rebounding motion of the vehicle body. Furthermore, as set out, working fluid in the pilot chamber PR is generally introduced through the pilot path PP via the multi-stage orifice Qp and return through the pilot return path PT via the lower section 42Ul of the control chamber 42U and via the multi-stage orifice Pr. As long as the fluid flow in the pilot return path PT is not disturbed and thus steady. The most upstream side orifice Pr' is mainly effective for restricting the fluid flow. Therefore, magnitude of flow restriction is relatively small so as to provide sufficient response characteristics in reduction of the pilot pressure. On the other hand, when the working fluid flowing from the control chamber 42U confluence with the working fluid from the pilot chamber PR, back pressure is produced in the drain port 54r, the fluid flowing through the pilot return path PT is disturbed and thus becomes unstable. This tends to cause serving of the pressurized fluid from the drain port 54r to the pilot chamber PR. In such case, all of the orifices in the multi-stage orifice Pr is effective to create greater flow restriction that than for the steady flow. This avoid influence of the back pressure created in the drain port 54r.

Similarly, in response to the bounding stroke of the piston 26c, the valve spool 52 is shifted up and down to absorb bounding energy and maintains the fluid pressure in the working chamber 26d of the hydraulic cylinder 26 at the neutral pressure.

On the other hand, when the anti-rolling suspension control is taken place in response to the lateral acceleration exerted on the vehicle body, the control signal current value is derived on the basis of the magnitude of the lateral acceleration monitored by the lateral acceleration sensor 23. Generally, in order to suppress rolling motion of the vehicular body, the fluid pressure in the working chamber 26d of the hydraulic cylinder 26 which is provided for the suspension mechanism at the side where the vehicular height is lowered across the neutral position, is increased to suppress lowering motion of the vehicle body. On the other hand, the fluid pressure in the working chamber 26d of the hydraulic cylinder 26 which is provided for the suspension mechanism at the side where the vehicular height is risen across the neutral position, is decreased to suppress rising motion of the vehicle body. Therefore, in order to control the pressures in the working chambers 26d of the both side hydraulic cylinders 26, control signal current values are increased and creased across the neutral value I_(N).

For example, when rolling motion is caused by left turn of the vehicle, control current for the actuators 29 of the pressure control valves 28 controlling the fluid pressures in the front-right and T rear-right hydraulic cylinders 26FR and 26RR are to be increased to be greater than the neutral current I_(N), and the control current for the actuator of the pressure control valves 28 controlling the fluid pressures in the front-left and rear-left hydraulic cylinders 26FL and 26RL are to be decreased to be smaller than the neutral current I_(N). By the control current supplied to respective actuators 29, the proportioning solenoid coils 68 are energized at the magnitudes corresponding to the control signal currents to place the poppet valves 48 at respective corresponding positions. By variation of the positions of the poppet valves 48, flow restriction magnitude at respective communication path openings 46A is varied to vary the pilot pressures Pp in the pilot chamber PR. As set forth, since the fluid pressures in the working chambers 26d become equal to the pilot pressures Pp, the suspension characteristics at respective hydraulic cylinders 26 can be adjusted.

Anti-pitching, bouncing suppressive suspension control can be performed substantially in the same manner to that discussed with respect to the anti-rolling control.

While the present invention has been disclosed in terms of the preferred embodiment in order to facilitate better understanding of the invention, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principle of the invention set out in the appended claims.

As will be appreciated from the discussion given hereabove, the pressure control valve 28, according to the present invention, is so arranged as to direct the axis of the valve bore 44 parallel to the longitudinal axis of the vehicle body. The longitudinal acceleration to be exerted on the vehicular body is much smaller than the lateral acceleration and vertical acceleration exerted on the vehicle body. Therefore, by arranging the pressure control valve 28 so that the poppet valve 48 and the valve spool 52 thrustingly move in longitudinal direction, influence of the externally applied acceleration can be minimized. Therefore, the objects and advantages can be successfully achieved. 

What is claimed is:
 1. A hydraulic circuit for an actively controlled suspension system which includes a pressure control valve unit connected to a working chamber in a hydraulic cylinder disposed between a vehicle body and a suspension member rotatable supporting a road wheel, said pressure control valve having an inlet port, a pressure control port and a drain port and connected to said working chamber via said pressure control port for adjusting fluid pressure in said working chamber in order to control suspension characteristics according to the vehicle driving condition, said hydraulic circuit being associated with said pressure control valve unit for supplying a pressurized fluid, which hydraulic circuit comprising:a fluid pressure source unit for generating a pressurized fluid and including a high pressure section for supplying said pressurized fluid and a low pressure section for storing the working fluid; a supply line connecting said high pressure section of said fluid pressure source unit to said inlet port of said pressure control valve unit for supplying said pressurized fluid; a drain line connecting said drain port of said pressure control valve unit to said low pressure section of said fluid pressure source unit; a valve means disposed within said drain line and variable of operational mode depending upon a line pressure in said supply line, said valve means operating in a first mode to perform pressure relief operation for draining a fluid pressure in said drain line higher than a predetermined pressure while said fluid pressure in the drain line upstream thereof is higher than said line pressure, and operating in a second mode to perform checking operation for switching valve position between open position and closed position when said line pressure in said supply line varies across said predetermined pressure.
 2. A hydraulic circuit as set forth in claim 1, wherein said pressure control valve unit is adapted to control fluid pressure in said working chamber in a range between a predetermined maximum pressure and a predetermined minimum pressure across a predetermined neutral pressure, at which the relative distance between said vehicle body and said suspension member is maintained at a predetermined standard distance at a standard load condition, and said valve means is set said predetermined pressure at a value corresponding to said neutral pressure of said pressure control valve unit.
 3. A hydraulic circuit as set forth in claim 2, wherein said valve means comprises a valve means having a first and second components which are operative independent of each other in said first mode and cooperative with each other in said second mode.
 4. A hydraulic circuit as set forth in claim 3, wherein said valve means comprises a valve housing defining a valve bore in which said first and second components are thrustingly disposed, said first and second components being cooperative with the inner periphery of said valve housing to define a first chamber communicated with said supply line via a pilot line, a second chamber disposed between said drain port of said pressure control valve unit and said low pressure section of said pressure source unit, said second chamber being defined between said first and second components and said first chamber is defined at a side of said first component remote from said second chamber so that the operational mode of said valve means is switched between said first mode and said second mode depending upon the force balance of the fluid force exerted on both sides of said first component.
 5. A hydraulic circuit as set forth in claim 4, wherein said second component is associated with a initial force setting means acting on the side remote from said first chamber, said initial force setting means exerting said predetermined pressure to said second component.
 6. A hydraulic circuit as set forth in claim 5, wherein said initial force setting means comprises a mechanical spring.
 7. In a hydraulic circuit for an actively controlled suspension system which includes a pressure control valve unit connected to a working chamber in a hydraulic cylinder disposed between a vehicle body and a suspension member rotatable supporting a road wheel, said pressure control valve having an inlet port, a pressure control port and a drain port and connected to said working chamber via said pressure control port for adjusting fluid pressure in said working chamber in order to control suspension characteristics according to the vehicle driving condition, said hydraulic circuit being associated with said pressure control valve unit for supplying a pressurized fluid, and said hydraulic circuit including a fluid pressure source unit for generating a pressurized fluid and including a high pressure section for supplying said pressurized fluid and a low pressure section for storing the working fluid, a supply line connecting said high pressure section of said fluid pressure source unit to said inlet port of said pressure control valve unit for supplying said pressurized fluid, a drain line connecting said drain port of said pressure control valve unit to said low pressure section of said fluid pressure source unit, a valve means, disposed within said drain line, comprising:first means, associated with said valve means, for varying operational mode of said valve means depending upon a line pressure in said supply line, said first means operating said valve means in a first mode while said fluid pressure in the drain line upstream of said valve means is higher than said line pressure, and operating in a second mode while said pressure in said drain line upstream of said valve means is lower than or equal to the pressure in said supply line; a second means, active during said first mode of said valve means for performing pressure relief operation for draining a fluid pressure in said drain line higher than a predetermined pressure; and a third means, active during said second mode of said valve means, for performing fluid flow checking operation for switching valve position between open position and closed position.
 8. A valve means as set forth in claim 7, wherein said pressure control valve unit is adapted to control fluid pressure in said working chamber in a range between a predetermined maximum pressure and a predetermined minimum pressure across a predetermined neutral pressure, at which the relative distance between said vehicle body and said suspension member is maintained at a predetermined standard distance at a standard load condition, and said valve means is set said predetermined pressure at a value corresponding to said neutral pressure of said pressure control valve unit.
 9. A valve means as set forth in claim 8, wherein said first means comprises a valve member having a first and second components which are operative independent of each other in said first mode and cooperative with each other in said second mode.
 10. A valve means as set forth in claim 8, wherein said second means is active on said first component for shifting the latter depending upon the fluid pressure in said drain line for relieving fluid pressure in excess of said predetermined pressure.
 11. A valve means as set forth in claim 9, wherein said third means is active on said first and second components in a condition cooperating to each other, for establishing fluid communication between said drain port of said pressure control valve unit and said low pressure section of said pressure source unit when said pressure in said supply line is higher than said predetermined pressure and blocking fluid communication between said drain port of said pressure control valve unit and said low pressure section of said pressure source unit when said pressure in said supply line is lower than or equal to said predetermined pressure.
 12. A valve means as set forth in claim 10, wherein said third means is active on said first and second components in a condition cooperating to each other, for establishing fluid communication between said drain port of said pressure control valve unit and said low pressure section of said pressure source unit when said pressure in said supply line is higher than said predetermined pressure and blocking fluid communication between said drain port of said pressure control valve unit and said low pressure section of said pressure source unit when said pressure in said supply line is lower than or equal to said predetermined pressure.
 13. An actively controlled suspension system comprising:a hydraulic cylinder disposed between a vehicle body and a suspension member rotatable supporting a road wheel and defining a working chamber filled with a hydraulic working fluid; a hydraulic circuit includes a pressure control valve unit connected to said working chamber, said pressure control valve having an inlet port, a pressure control port and a drain port and connected to said working chamber via said pressure control port for adjusting fluid pressure in said working chamber in order to control suspension characteristics according to the vehicle driving condition; a fluid pressure source unit for generating a pressurized fluid and including a high pressure section for supplying said pressurized fluid and a low pressure section for storing the working fluid; a supply line connecting said high pressure section of said fluid pressure source unit to said inlet port of said pressure control valve unit for supplying said pressurized fluid; a drain line connecting said drain port of said pressure control valve unit to said low pressure section of said fluid pressure source unit; a valve means disposed within said drain line and variable of operational mode depending upon a line pressure in said supply line, said valve means operating in a first mode to perform pressure relief operation for draining a fluid pressure in said drain line higher than a predetermined pressure while said fluid pressure in the drain line upstream thereof is higher than said line pressure, and operating in a second mode to perform checking operation for switching valve position between open position and closed position when said line pressure in said supply line varies across said predetermined pressure.
 14. An actively controlled suspension system as set forth in claim 13, wherein said pressure control valve unit is adapted to control fluid pressure in said working chamber in a range between a predetermined maximum pressure and a predetermined minimum pressure across a predetermined neutral pressure, at which the relative distance between said vehicle body and said suspension member is maintained at a predetermined standard distance at a standard load condition, and said valve means is set said predetermined pressure at a value corresponding to said neutral pressure of said pressure control valve unit.
 15. An actively controlled suspension system as set forth in claim 14, wherein said valve means comprises a valve means having a first and second components which are operative independent of each other in said first mode and cooperative with each other in said second mode.
 16. An actively controlled suspension system as set forth in claim 15, wherein said valve means comprises a valve housing defining a valve bore in which said first and second components are thrustingly disposed, said first and second components being cooperative with the inner periphery of said valve housing to define a first chamber communicated with said supply line via a pilot line, a second chamber disposed between said drain port of said pressure control valve unit and said low pressure section of said pressure source unit, said second chamber being defined between said first and second components and said first chamber is defined at a side of said first component remote from said second chamber so that the operational mode of said valve means is switched between said first mode and said second mode depending upon the force balance of the fluid force exerted on both sides of said first component.
 17. An actively controlled suspension system as set forth in claim 16, wherein said second component is associated with a initial force setting means acting on the side remote from said first chamber, said initial force setting means exerting said predetermined pressure to said second component.
 18. An actively controlled suspension system as set forth in claim 16, wherein said initial force setting means comprises a mechanical spring. 